diff --git a/packages/api/src/activities/activity.spec.ts b/packages/api/src/activities/activity.spec.ts
index dd1b7767a..a423accb8 100644
--- a/packages/api/src/activities/activity.spec.ts
+++ b/packages/api/src/activities/activity.spec.ts
@@ -33,9 +33,7 @@ describe('Activity', () => {
         conversation: chat,
       })
       .withRecipient(bot)
-      .withServiceUrl('http://localhost')
-      .withTimestamp(new Date())
-      .withLocalTimestamp(new Date());
+      .withServiceUrl('http://localhost');
 
     expect(activity.id).toEqual('1');
     expect(activity.type).toEqual('test');
@@ -51,8 +49,20 @@ describe('Activity', () => {
 
     expect(activity.recipient).toEqual(bot);
     expect(activity.serviceUrl).toEqual('http://localhost');
-    expect(activity.timestamp).toBeDefined();
-    expect(activity.localTimestamp).toBeDefined();
+  });
+
+  it('should preserve timestamp strings from constructor data', () => {
+    const timestamp = '2026-06-04T21:00:00.000Z';
+    const localTimestamp = '2026-06-04T14:00:00.000-07:00';
+
+    const activity = new Activity({
+      type: 'test',
+      timestamp,
+      localTimestamp,
+    });
+
+    expect(activity.timestamp).toEqual(timestamp);
+    expect(activity.localTimestamp).toEqual(localTimestamp);
   });
 
   it('should build from interface', () => {
diff --git a/packages/api/src/activities/activity.ts b/packages/api/src/activities/activity.ts
index d23162ed1..588c05579 100644
--- a/packages/api/src/activities/activity.ts
+++ b/packages/api/src/activities/activity.ts
@@ -34,7 +34,7 @@ export interface IActivity<T extends string = string> {
   /**
    * Contains the date and time that the message was sent, in UTC, expressed in ISO-8601 format.
    */
-  timestamp?: Date;
+  timestamp?: string;
 
   /**
    * A locale name for the contents of the text field.
@@ -50,7 +50,7 @@ export interface IActivity<T extends string = string> {
    *
    * For example, 2016-09-23T13:07:49.4714686-07:00.
    */
-  localTimestamp?: Date;
+  localTimestamp?: string;
 
   /**
    * Contains an ID that uniquely identifies the channel. Set by the channel.
@@ -138,7 +138,7 @@ export class Activity<T extends string = string> implements IActivity<T> {
   /**
    * Contains the date and time that the message was sent, in UTC, expressed in ISO-8601 format.
    */
-  timestamp?: Date;
+  timestamp?: string;
 
   /**
    * A locale name for the contents of the text field.
@@ -154,7 +154,7 @@ export class Activity<T extends string = string> implements IActivity<T> {
    *
    * For example, 2016-09-23T13:07:49.4714686-07:00.
    */
-  localTimestamp?: Date;
+  localTimestamp?: string;
 
   /**
    * Contains an ID that uniquely identifies the channel. Set by the channel.
@@ -302,21 +302,11 @@ export class Activity<T extends string = string> implements IActivity<T> {
     return this;
   }
 
-  withTimestamp(value: Date) {
-    this.timestamp = value;
-    return this;
-  }
-
   withLocale(value: string) {
     this.locale = value;
     return this;
   }
 
-  withLocalTimestamp(value: Date) {
-    this.localTimestamp = value;
-    return this;
-  }
-
   withChannelData(value: ChannelData) {
     const merged: ChannelData = { ...this.channelData, ...value };
 
diff --git a/packages/devtools/src/stores/ChatStore.ts b/packages/devtools/src/stores/ChatStore.ts
index 5f77ea2ae..4bf53947a 100644
--- a/packages/devtools/src/stores/ChatStore.ts
+++ b/packages/devtools/src/stores/ChatStore.ts
@@ -40,6 +40,10 @@ interface MessageBase {
   createdDateTime: string;
 }
 
+const toUTCString = (value?: string | Date) => {
+  return (value instanceof Date ? value : new Date(value || Date.now())).toUTCString();
+};
+
 const createMessageBase = (
   event: ActivityEvent<IMessageActivity | ITypingActivity>
 ): MessageBase => {
@@ -66,7 +70,7 @@ const createMessageBase = (
           }
         : undefined,
     },
-    createdDateTime: (event.body.timestamp || new Date()).toUTCString(),
+    createdDateTime: toUTCString(event.body.timestamp),
   };
 };
 
@@ -303,7 +307,7 @@ export const useChatStore = create<ChatStore>()(
           message.body.textContent = event.body.text;
         }
 
-        message.lastModifiedDateTime = (event.body.timestamp || new Date()).toUTCString();
+        message.lastModifiedDateTime = toUTCString(event.body.timestamp);
         state.put(state.chat.id, message);
         return state;
       },
diff --git a/packages/devtools/src/utils/create-feedback.ts b/packages/devtools/src/utils/create-feedback.ts
index 590c55576..c757d73cd 100644
--- a/packages/devtools/src/utils/create-feedback.ts
+++ b/packages/devtools/src/utils/create-feedback.ts
@@ -49,7 +49,7 @@ export const createFeedbackActivity = ({
       conversationType: conversation.conversationType || 'personal',
     },
     recipient: invokeRecipient,
-    localTimestamp: new Date(),
+    localTimestamp: new Date().toISOString(),
     locale,
     value: {
       actionName: 'feedback' as const,